tty: serial: atmel: Call dma_async_issue_pending()

[ Upstream commit 4f4b9b5895 ]

The driver wrongly assummed that tx_submit() will start the transfer,
which is not the case, now that the at_xdmac driver is fixed. tx_submit
is supposed to push the current transaction descriptor to a pending queue,
waiting for issue_pending to be called. issue_pending must start the
transfer, not tx_submit.

Fixes: 34df42f59a ("serial: at91: add rx dma support")
Fixes: 08f738be88 ("serial: at91: add tx dma support")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20211125090028.786832-4-tudor.ambarus@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/tty/serial/atmel_serial.c

@@ -1009,6 +1009,8 @@ static void atmel_tx_dma(struct uart_port *port)
 				atmel_port->cookie_tx);
 			return;
 		}
+
+		dma_async_issue_pending(chan);
 	}
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
@@ -1269,6 +1271,8 @@ static int atmel_prepare_rx_dma(struct uart_port *port)
 		goto chan_err;
 	}
 
+	dma_async_issue_pending(atmel_port->chan_rx);
+
 	return 0;
 
 chan_err: